Osteology of a large allosauroid theropod from the Upper Jurassic (Tithonian) Morrison Formation of Colorado, USA

Doi: 10.5604/17313708 .1130141

Osteology of a large allosauroid theropod from the Upper Jurassic (Tithonian) Morrison Formation of Colorado, USA

Sebastian G. DALMAN

Key words: dinosaur, Theropoda, Allosauridae, Morrison Formation, Upper Jurassic, Colorado.

Abstract. Two partial skeletons of allosaurid theropods belonging to an adult and a juvenile from the Upper Jurassic (Tithonian) Morrison Formation of McElmo Canyon in Montezuma County, southwestern Colorado, were discovered in 1953 by the late Joseph T. Gregory and David Techter. The adult specimen consists of several isolated cranial and postcranial skeletal elements that are exceptionally well-pre- served and include the left premaxilla, maxilla, dentary, teeth, quadratojugal, two caudal vertebrae, pubic peduncle, ischium, proximal tibia, a nearly complete left foot, and several isolated teeth, whereas the juvenile specimen is represented by the distal portion of the right dentary and a fragmentary splenial. The specimens represent a new species of Allosaurus, here named Allosaurus lucasi, which differs from Allosaurus fragilis by having a relatively short premaxilla and robust quadratojugal with short jugal process and a short quadrate process of the quadratojugal that is at the same level as the rostral quadratojugal ramus. The presence of a new species of Allosaurus in the Tithonian of North America provides further evidence of the taxonomic and morphological diversity of the Allosauridae clade and their continuous evolutionary success, which extended to the Cretaceous.

1Department of Geosciences, Fort Hays State University, 600 Park Street, Hays, KS 67601, USA; e-mail: sebastiandalman@yahoo.com:

Corresponding address: 104 Johnson Dr.1002 Chicopee, MA 01022, USA

largest predators that lived during the Late Jurassic and Cre- taceous. Their remains are also known from Asia, Africa, and Europe (Pérez-Moreno et al., 1999; Madsen, Welles, 2000;

Naish, 2003; Mateus, 2006; Mateus et al., 2006). I add to knowledge of North American Late Jurassic allosaurid diver- sity two specimens, here named Allosaurus lucasi sp. nov., representing adult and juvenile individuals, that were discov- ered in 1953 by the late Joseph T. Gregory and David Techter in McElmo Canyon in Montezuma County, Colorado, near the top of the Upper Jurassic Morrison Formation (Tithonian). The specimens were found in a hard conglomer- atic matrix, which still encases some of the bones. The adult specimen consists of several cranial and postcranial ele- ments, some of which are fragmentary and others, such as the left pes, are nearly complete, whereas the juvenile is known only from a fragmentary right distal dentary and par- tial splenial. The presence of a new species of Allosaurus in INTRODUCTION

During the Late Jurassic in North America, Allosaurus was the numerically dominant large-bodied theropod, which shared the same ecosystems with other large-bodied thero- pods such as Torvosaurus tanneri (Galton, Jensen, 1979;

Jensen, 1985; Britt, 1991), Ceratosaurus dentisulcatus, C. magni cornis, C. nasicornis, and Saurophaganax maximus (Gilmore, 1920; Chure, 1995; Madsen, Welles, 2000; Foster, 2003, 2007). Numerous skeletal remains of Allosaurus are found throughout the entire Morrison Formation in modern day Colorado, Montana, New Mexico, Oklahoma, South Da- kota, Utah, and Wyoming.

The most complete and the best known species of Allo­

saurus are Allosaurus atrox (formerly Creosaurus atrox;

Marsh, 1878), A. fragilis (Madsen, 1993), and A. jimmadseni

(Chure, 2000). The Allosauridae clade includes some of the

the Tithonian of North America provides further evidence of the taxonomic and morphological diversity of the Allosauri- dae clade and their continuous evolutionary success, which extended to the Cretaceous. In this paper, YPM refers to the Yale Peabody Museum of Natural History.

GEOLOGICAL BACKGROUND

Field notes (July 20 to July 23, 1953) of Joseph T. Greg- ory of the Yale Peabody Museum of Natural History, New Haven, Connecticut, USA, indicate that the allosaurid speci- mens described here (YPM VP 57589 and YPM VP 57726) were collected west of Cortez in southwestern Colorado at

McElmo Canyon from the top of the Morrison Formation (Fig. 1). The lithology at the site is largely a conglomerate with a mix of fine-grained sandstone indicating a fluvial en- vironment. The uppermost sediments in McElmo Canyon where the allosaurids were collected are included in the Brushy Basin Member of the Morrison Formation. Kowallis et al. (1998) reported that sanidine ages from the Brushy Ba- sin Member in southwestern Colorado range from 150.33

±0.27 Ma to 147.82 ±0.63 Ma, which indicate an age of Kimmeridgian to Tithonian. Bralower et al. (1990), Harland et al. (1990), Obradovich (1993), O’Sullivan (1997), and Turner and Peterson (2004) also indicated that the Brushy Basin Member in southwestern Colorado is Kimmeridgian to Early Tithonian in age. The age of the Morrison Forma-

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Fig. 1. Map of southern Colorado, USA showing the location of McElmo Canyon in which the Allosaurus lucasi sp. nov. (YPM VP 57589) was found (modified from O’Sullivan, 1997)

tion is similar to the Solnhofen Limestone in Germany, the Lou rinhã and Alcobaça formations in Portugal, and the Tenda- guru Formation in Tanzania (Mateus et al., 2006; Foster, 2007).

SYSTEMATIC PALEONTOLOGY Saurischia Seeley, 1887 Theropoda Marsh, 1881 Tetanurae Gauthier, 1986 Carnosauria von Huene, 1920

Allosauridae Marsh, 1878

Holotype: YPM VP 57589, partial skeleton of an adult consist- ing of cranial, dental, and postcranial elements.

Included species: The type material is only known from type species Allosaurus lucasi sp. nov.

Included specimens: YPM VP 57726, posterior end of the right dentary and part of the right splenial belonging to a juvenile.

Etymology: The species name honors Spencer G. Lucas in re co- gnition of his extensive contributions to vertebrate paleonto logy.

Type locality, horizon and age: McElmo Canyon, Montezuma County, Colorado, USA; Brushy Basin Member of the Morrison Formation Upper Jurassic (Tithonian).

Diagnosis: Large-bodied allosauroid theropod with the follow- ing autapomorphies: strongly reduced length of premaxilla;

short and deep maxilla; quadratojugal with reduced jugal pro- cess and quadrate process; the ventral margins of the rostral quadratojugal ramus and the quadrate process of the quadratoju- gal form a single line; lateral condyle of the tibia strongly re- moved posteriorly, whereas in Allosaurus fragilis and Sauro­

phaganax maximus the condyle is more centered and occupies almost half the length of the tibial head.

Remarks. – All specimens described here were found to- gether at the same locality. They exhibit the same morpho- logy, which suggests that they belong to the same species, but to theropods of different sizes and age groups.

DESCRIPTION AND COMPARISONS

The type specimen of Allosaurus lucasi (YPM VP 57589) consists of well-preserved but fragmentary cranial and postcranial skeletal elements.

CRANIAL SKELETON

Premaxilla. The left premaxilla of Allosaurus lucasi is well-preserved (Fig. 2). It is approximately 11 cm long and 11 cm high. In several basal tetanuran theropods such as

Allosaurus, Monolophosaurus, Neovenator, and Sinraptor, the premaxilla is longer than tall not including the nasal pro- cess (Currie, Zhao, 1993; Zhao, Currie, 1993; Brusatte et al., 2008a, b, 2010; Eddy, Clarke, 2011) (Fig. 3). As in other examples of Allosaurus, there are five teeth in the premaxilla of A. lucasi. The number of premaxillary teeth in basal and more derived allosauroid theropods vary and is five in A. fra­

gilis, A. jimmadseni, and four in Sinraptor dongi, whereas in derived allosauroids, the carcharodontosaurid theropods such as Acrocanthosaurus atokensis and Carcharodontosau­

rus saharicus, the premaxillary tooth count is four, and most likely it was the same in less complete taxa for which the premaxillae are unknown such as C. iguidensis, Eocarcharia dinops, Giganotosaurus carolinii, Kelmayisaurus petrolicus, Mapusaurus rosea, Shaochilong maortuensis, and Tyran­

notitan chubutensis.

In Allosaurus lucasi the medial side of the bone is ce- mented to the conglomeratic matrix; covering most of the important features. However, in the future the specimen will be better prepared to reveal these features. The body of the premaxilla is slightly trapezoidal. The anteroventral margin of the premaxilla is inclined vertically and forms a low-an- gle snout, whereas the posterior margin is slightly angled posteriorly. The premaxillary angle that is formed between the ventral and anterior margins is 55° in A. lucasi, whereas in A. fragilis and Ceratosaurus magnicornis and Torvosau­

rus tanneri the premaxillary angle is slightly higher (Britt, 1991; Madsen, Welles, 2000). Allosaurus, Ceratosaurus, and Torvosaurus are distinguished from each other based on various premaxillary angle measurements (Britt, 1991). The alveoli for the premaxillary teeth are slightly elliptical in shape. The length of each of the alveoli is 2 cm. The angle, as measured between the symphysis and the center of the

A B

Fig. 2. Left premaxilla of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian), McElmo Canyon, Montezuma County,

Colorado, USA

A. Lateral view. B. Ventral view showing the premaxillary tooth row and partially preserved teeth. Abbreviations: m – maxilla contact; na – bony naris;

lpm1 to lpm5 – premaxillary teeth

most anterior and most distal alveoli, is 20° in A. lucasi, whereas in A. fragilis the angle is between 25° and 30°. The consequence of this small angle in A. lucasi is that the snout is relatively narrow. Similar conditions are also present in other large theropods such as Acrocanthosaurus, Carcharo­

dontosaurus, and Giganotosaurus (Currie, Carpenter, 2000;

Eddy, Clarke, 2011).

The lateral surface of the premaxillary body of Allosau­

rus lucasi is smooth and lacks any type of ornamentation.

Additionally, there is no evidence of the neurovascular fo- ramina due to extensive weathering of the lateral surface of the bone.

Maxilla. The left maxilla of Allosaurus lucasi is partially preserved, missing most of the posterior portion (Fig. 4).

The body of the preserved maxilla is triangular, and its length is approximately 17.7 cm. However, when compared to other large-bodied non-avian theropods (e.g., Acrocantho­

saurus, Allosaurus, and Torvosaurus) the original length of the maxilla of A. lucasi would have been approximately 35.4 cm, whereas the entire length of the skull would have been approximately 77.5 cm. These estimates are based on the length proportions of other large-bodied theropods such as Acrocanthosaurus atokensis and Allosaurus fragilis.

The maxilla is thick anteriorly and becomes thinner to- wards the posterior end, and it is slightly convex with a slight upturn in the anterior one-third, which is similar to that of Torvosaurus tanneri (Britt, 1991) (Fig. 5). The premaxilla- maxilla contact is not very clear; however, under closer ex- amination it is discernible. The body of the maxilla in Allo­

saurus lucasi is relatively deep in the anterior region.

It shows some similarities to the maxilla of Acrocanthosau­

rus. As in Acrocanthosaurus, the subnarial foramen in A. lu­

casi is absent, but it is present in A. fragilis. The subnarial foramen is also absent in other allosauroid theropods such as Carcharodontosaurus, Giganotosaurus, Neovenator, Shao­

chilong, and Sinraptor, but also in the abelisaurid theropods such as Aucasaurus, Carnotaurus, Ekrixinatosaurus, Eoa­

belisaurus, Indosuchus, Majungasaurus, and Rugops.

Basal tetanurans in general possess a distinctly shaped trian- gular anterior ascending ramus and a dorsal anterodorsal pro- cess that contacts the nasal (Brusatte et al., 2010b). In Allosau­

rus lucasi only the base of the ascending ramus is preserved.

The tooth row in Allosaurus lucasi is not well-preserved, even though some partial teeth are preserved. Therefore, the exact maxillary tooth count cannot be determined at present.

The number of the maxillary tooth count in basal allosauroid A

E D

B C

Fig. 3. Comparison of premaxillae of the Jurassic and Cretaceous basal tetanuran theropods

A. Allosaurus lucasi. B. Allosaurus fragilis. C. Monolophosaurus jiangi. D. Neovenator salerii. E. Sinraptor dongi. Premaxillae in lateral view. Fig B after Madsen (1993); Fig. c after Zhao, currie (1993); Fig. D after Brusatte et al. (2008a); Fig. E after currie, Zhao (1994). not to scale

theropods is variable: 16 in A. jimmadseni, and 15 in A. fra­

gilis and Sinraptor dongi. In carcharodontosaurid theropods the maxillary tooth count is 15 in Acrocanthosaurus atoken­

sis and Eocarcharia dinops, 14 in Carcharodontosaurus sa­

haricus, and 12 in Giganotosaurus carolinii, Mapusaurus rosea, and Shaochilong maortuensis. It is possible that the variation in tooth count in these taxa represents an onto- genetic variation.

The sandstone matrix on which the maxilla of Allosaurus lucasi rests contains a mix of bones from the skull, which in- clude two partially preserved alveoli and a single preserved root of a tooth. Some of the flat bones exposed on the surface of the matrix are either parts of the maxilla or some other skull bones.

Quadratojugal. The left quadratojugal of Allosaurus lu­

casi is well preserved (Fig. 6). As in other theropods the bone is L-shaped and possesses a relatively short rostral quadratojugal ramus. The bone is split in half vertically, and the two parts are mirror images of each other encased in hard sandstone matrix.

The length of the preserved quadratojugal is 16 cm, and its height is 14.5 cm. The jugal process of the quadratojugal is broken off and missing; however, when reconstructed the jugal process of A. lucasi appears much shorter than that of A. atrox, A. fragilis and A. jimmadseni. The angle between the dorsal quadratojugal ramus and the rostral quadratojugal ramus in A. lucasi is approximately 90º, whereas in Allosaurus

10 cm

pm

eaof

eaof

M

M ascr

PM PM PM

A

C B

Fig. 4. Left premaxilla and maxilla of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon,

Montezuma County, Colorado, USA

A. Lateral view. B. Dorsal view. C. Ventral view showing the maxillary tooth row and partially preserved teeth. Abbreviations: ascr – ascending ramus of maxilla; eaof – margin of external antorbital fenestra; m – maxilla; Pm – premaxilla; pm – premaxillary contact

A

B

Fig. 5. Comparison of the maxillae of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) of McElmo Canyon, Montezuma County, Colorado, and Torvosaurus tanneri (BYUVP 9122;

Britt, 1991) from the Morrison Formation (Tithonian) Dry Mesa Quarry, Colorado, USA. (Torvosaurus tanneri maxilla after Britt, 1991) not to scale

the angle is 75º. The dorsal quadratojugal ramus in A. lucasi is relatively straight, whereas in Allosaurus it is bent for- ward. Similar conditions to that of A. lucasi are observable in the reconstructed skull of Torvosaurus tanneri (Britt, 1991). However, the quadratojugal of T. tanneri has yet to be collected; therefore, the reconstruction and its position in the skull is based on another bone, the quadrate (Britt, 1991).

The rostral ramus in A. lucasi is deepest caudally and tapers rostrally. Taxonomic variation is in large part associated with the relative shape and size of the dorsal and the rostral rami (Sampson, Witmer, 2007). Within basal theropods the con- tact between the quadratojugal and squamosal is variable (Sampson, Witmer, 2007). The dorsal ramus in Herrerasau­

rus, for example, is twice the width of the rostral ramus and does not have a clear connection with the squamosal (Sere- no, Novas, 1993). However, taxa such as Ceratosaurus and Eoraptor have both rami relatively slender and narrow quad- ratojugal-squamosal contact (Sampson, Witmer, 2007). In contrast, in albertosaurine (e.g., Albertosaurus and Gorgo­

saurus) and in tyrannosaurine tyrannosaurids (e.g., Dasple­

tosaurus, Tarbosaurus, and Tyrannosaurus) the dorsal rami are extensively expanded, and have a broad, rostrally pro- jected contact for the squamosal, which form a large latero-

temporal fenestra (Carr, 1999; Carr, Williamson, 2004;

Sampson, Witmer, 2007). As in Allosaurus fragilis (Madsen, 1993), the squamosal contact in A. lucasi is similarly broad, and the laterotemporal fenestra in both taxa may have been also of similar shape.

The length of the quadrate process of the quadratojugal in A. lucasi is 4.5 cm, and the height is 6 cm. The process is broader ventrally and forms a triangular slot for the contact with the quadrate. The dorsal quadratojugal ramus is approx- imately 13 cm tall. The process is broader ventrally and nar- rower dorsally, as in other theropods. The ventral side of the rostral quadratojugal ramus is in line with the quadrate pro- cess of the quadratojugal. A similar condition is also present in A. jimmadseni (Chure, 2000).

Quadrate. The left quadrate of Allosaurus lucasi is in- complete, however, several features, which are not well-pre- served, can be recognized, including the pterygoid flange, pterygoid ramus, quadratojugal contact, quadrate shaft, and quadratojugal ramus of quadrate (Fig. 7). The bone is split in half and filled with hard sandstone matrix. The height of the bone is 13 cm. The pterygoid flange is incomplete; however, A

D C

B

10 cm sq

dqjr qp ltf

rqjr

Fig. 7. Left quadrate of Allosaurus lucasi sp. nov. (YPM VP 57589) (above) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA compared with the left quadrate of

Allosaurus fragilis (YPM VP 14554) (below)

A. Lateral. B. Medial. C. Rostral views. Abbreviations: ptr – pterygoid ramus of quadrate; qj – quadratojugal contact; qs – quadrate shaft

Fig. 6. Left quadratojugal of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma

County, Colorado, USA

A and B. Two halves of the quadratojugal in sandstone matrix; C and D. Two halves of the quadratojugal (digitally removed matrix). Abbreviations: dqjr – dorsal quadratojugal ramus; ltf – laterotemporal fenestra; qp – quadrate process of quadratojugal; rqjr – rostral quadratojugal ramus; sq – squamosal contact

A B C

5 cm qj

ptr qs

it appears when reconstructed that it was anteroposteriorly long. As in other large-bodied theropods (Currie, Zhao, 1993; Madsen, 1993; Currie, 2006) the ventral margin of the pterygoid flange in A. lucasi was most likely curled medial- ly. Most of the anterior and posterior sides of the lateral and medial surfaces are missing, including parts of the pterygoid process. The surface of the lateral side is smooth and lacks any ornamentation. The medial surface exhibits shallow sculpting in the form of parallel lines. These parallel lines represent muscle scars for the attachment of the posterior M. adductor mandibulae (Molnar, 2008).

The quadrate of Allosaurus lucasi is relatively tall, as in other medium- and large-bodied theropods, including Cera­

tosaurus and Sinraptor (Currie, Zhao, 1993; Madsen, Welles, 2000; Rauhut, 2003; Currie, 2006), and most abeli- saurids (Bonaparte, Novas, 1985; Bonaparte et al., 1990;

Coria et al., 2002; Sampson, Witmer, 2007; Canale et al., 2008; Pol, Rauhut, 2012). Although, the quadrate condyle for the articulation with the quadrate contact on the quadrate process of the quadratojugal and for the articulation with the lower jaw is missing, the posterior end of the bone appears to be very slender, whereas in A. fragilis the structure is ro- bust. In medial view the quadrate of A. lucasi is morphologi- cally similar to that of Sinraptor dongi (Currie, Zhao, 1993).

A distinct feature observed in allosauroid theropods (e.g., Acrocanthosaurus, Allosaurus, Giganotosaurus, Mapusau­

rus, Saurophaganax, Shaochilong, and Sinraptor) is the presence of a quadratic foramen (Currie, Zhao, 1993; Chure, 1995; Coria, Salgado, 1995; Coria, Currie, 2006; Brusatte, Sereno, 2008; Eddy, 2008; Brusatte et al., 2009, 2010; Eddy, Clarke, 2011). Because of the poor preservation of the bone it is unclear whether the quadrate of A. lucasi possessed the quadratic foramen. Other non-avian theropods such as Cera­

tosaurus, Coelophysis, and abelisaurids, including Torvosau­

rus all lack quadratic foramina (Currie, 2006).

Dentaries. There are two partially preserved right den- taries referred to Allosaurus lucasi. One dentary belongs to an adult (YPM VP 57589), whereas the other smaller den- tary (YPM VP 57726) most likely represents a juvenile of A. lucasi (Fig. 8). According to the field notes of Gregory and Techter and their map of the specimen in situ (Fig. 9), these two dentaries were found in close proximity to each other and exhibit similar morphology. Therefore, it is tenable to suggest they belong to the same species.

A

C B

10 cm

Fig. 8. Dentaries of an adult of Allosaurus lucasi sp. nov. (YPM VP 57589;

on the left) and juvenile (YPM VP 57726; on the right) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado,

USA compared with the right dentary of Allosaurus fragilis (YPM VP 14554-8; on the left below)

A. Lateral. B. Medial. C. occlusal views; spl – splenial contact

Fig. 9. Field map of the bones of Allosaurus lucasi sp. nov. (YPM VP 57589 and YPM VP 57726) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA (after Gregory and Techter 1953, Yale Peabody Museum of Natural History Archives)

Dentary 1. A partial right dentary belongs to the holo- type of Allosaurus lucasi (YPM VP 57589). The length of the preserved dentary fragment is 15 cm long and its depth is 9 cm. The bone, in cross section, preserves a single unerupt- ed tooth. The tooth is split in half, exposing its cross section.

The lateral surface of the bone is smooth and featureless, whereas the medial surface is heavily eroded and filled with hard sandstone matrix. The interdental plates are fused. The height of the interdental plate measured from the top of the splenial contact is approximately 2.5 cm. The splenial con- tact is a convex structure with an overall height of 3 cm and contacts the splenial medially.

Dentary 2. A partially preserved distal end of the right dentary of Allosaurus lucasi (YPM VP 57726) belongs to a juvenile. The dentary preserves five alveoli, containing five partially preserved teeth. The alveoli are elliptical in shape.

The lateral surface of the bone is smooth and featureless and is similar to the dentary of an adult of A. lucasi. As in other theropods the dentary contacts the splenial medially. How- ever, the splenial contact is much deeper than it is in other species of Allosaurus; it narrows anteriorly, widens posteri- orly, and forms a V-shaped structure. The posteroventral por- tion of the dentary is much thinner than the anterior portion.

Splenial. A small elongated bone fragment identified as the right splenial (YPM VP 57726) most likely belongs to the juvenile individual (Fig. 10). The medial side of the bone has a shallow groove in the middle, which closely resembles the splenial contact. The ventral side of the bone has uniform thickness. When compared to the splenial of other large-bod- ied theropods the bone fragment (YPM VP 57726) most likely represents the anterior end of the splenial, as half way towards the posterior end the bone narrows uniformly and resembles that of other Allosaurus.

Teeth. YPM VP 57589 is a single well-preserved pre- maxillary tooth with root (Fig. 11). In lingual view, the tooth is convex. Most likely it is the fifth premaxillary tooth of the right premaxilla and resembles the premaxillary teeth of Al­

losaurus fragilis. Both anterior and posterior carinae are heavily eroded, and thus cannot provide important informa- tion on denticle density. The rostral carina of the premaxil- lary tooth in A. lucasi is oriented more medially, which is similar to that of A. fragilis. In labial view, the rostral side of the tooth has great convexity, whereas in the caudal side the tooth is flattened. The premaxillary tooth of A. lucasi pos- sesses a moderate curvature. The angle of the curvature in labial and lingual views is 20º, and is similar to A. fragilis.

5 cm A

B

Fig. 10. Splenial of juvenile of Allosaurus lucasi sp. nov.

(YPM VP 57726) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA A. Lateral. B. Medial

A B

2 cm

Fig. 11. Isolated premaxillary tooth of Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA A. labial. B. lingual views

Several isolated lateral teeth are partially preserved (Fig. 12). All teeth are laterally compressed and curved dis- tally and are either from the maxilla or dentary. Some of the teeth preserved in the matrix on the left premaxilla and max- illa lie are large, and resemble those of Allosaurus fragilis.

The degree of curvature can be determined in some teeth of A. lucasi. Sereno et al. (1998) suggested that tooth crowns with significant curvature are plesiomorphic in Theropoda, whereas teeth that lack curvature or with reduced curvature are considered the derived state and, therefore, a synapomor- phy of Spinosaurinae. Smith (2007) suggested that this char- acter represents a useful phylogenetic feature in theropods.

The maxillary teeth of A. lucasi possess moderate curva- tures. The anterior carina is oriented more on the lingual side, and the posterior carina is oriented on the labial side, which is similar to that of A. fragilis and to most other non- avian theropods.

The denticles on each lateral tooth are minute and similar to those of other allosauroid theropods such as Acrocantho­

saurus, Allosaurus, and Sinraptor. However, not all the teeth preserve denticles due to their poor preservation. Therefore, the denticle densities and morphology cannot be determined at present for any of the teeth.

AXIAL SKELETON

Caudal vertebrae. Two caudal vertebral centra are known for Allosaurus lucasi (Fig. 13). Both centra are com- plete and exceptionally well-preserved. The larger centrum resembles the 19

vertebra in the caudal series of A. fragilis

3 cm

Fig. 12. Isolated lateral teeth of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

A

C B

cf prz

nc ns

5 cm

Fig. 13. Distal caudal vertebrae of Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

A. Right lateral. B. Dorsal. C. Cranial views. Abbreviations: cf – chevron facet;

nc – neural canal; ns – neural spine; prz – prezygapophysis

(Madsen, 1993), and the smaller centrum is most likely the 25

or 28

in the caudal series (Madsen, 1993). As in A. fra­

gilis and A. jimmadseni (Madsen, 1993; Chure, 2000), the caudal centra of A. lucasi are amphicoelous. In A. fragilis there are approximately 50 caudal vertebrae that form the tail, but the total number is variable (Madsen, 1993). It is tenable that A. lucasi had a similar number of caudal verte- brae in the tail. However, at present it cannot be determined for certain because only two caudal centra are known.

APPENDICULAR SKELETON

Ilium. The YPM material includes the left pubic pedun- cle of the ilium of Allosaurus lucasi (Fig. 14). The antero- posterior length is 15 cm, height is 11 cm, and transverse width is 7 cm. The neck of the pubic peduncle is slender. The lateral surface of the bone is partially preserved, and the me- dial surface is heavily eroded. The articular surface for the

contact with the pubis is slightly convex and ends with a small protruding structure, which extends throughout the entire anteroposterior length of the bone. This characteristic structure is unknown in Allosaurus atrox, A. fragilis, A. jim­

madseni, Torvosaurus tanneri, and in other known thero- pods. In A. fragilis, the pubic peduncle is longer and less robust, whereas in T. tanneri it is transversely wide. The pos- terior region of the bone in A. lucasi is slightly concave, whereas in A. atrox, A. fragilis, A. jimmadseni, and T. tan­

neri the concavity is much deeper.

Pubis. Two isolated fragments represent the distal por- tion of the pubic shaft (Fig. 15). The most distal fragment of the pubis shaft is 9.5 cm wide and 7 cm long, whereas the other bone fragment is 7.5 cm wide and 10 cm long. In cross- section, both bone fragments have an elliptical shape. Unfor- tunately, the rest of the pubis is missing, and it is unclear if the pubis of A. lucasi closely resembled that of A. fragilis and A. jimmadseni. The pubis of A. fragilis is robust and the shaft itself is much thicker than that of A. lucasi.

Ischium. The YPM material includes a partially pre- served distal end of the left ischium (Fig. 16). The bone con- sists of a small portion of the distal shaft. A portion of the lateral surface of the ischium is encased in hard sandstone A

C B

10 cm

pp

Fig. 14. Pubic peduncle of the left ilium of A, Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA compared with ilium of B, Allosaurus fragilis (YPM VP 14554); C – Allosaurus atrox (YPM VP 1890) Note: ilia of A. fragilis and A. atrox in left lateral view. Abbreviations:

pp – pubic peduncle

5 cm

Fig. 15. Two isolated fragments of pubic shaft in lateral view of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation

(Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

matrix. The length of the preserved bone is 24 cm, and the anteroposterior width is 19 cm. The width of the shaft is 5.5 cm, whereas its distal end is 10.5 cm. The lateral surface of the bone is slightly convex, whereas its medial side is flat.

Tibia. The YPM material includes the proximal end of the left tibia (Fig. 17). The bone is large and robust, but is missing most of the shaft and is slightly compressed medio- laterally. The proximal articular end has an overall length of 18 cm. The bone is missing a significant portion of the cnemial crest, and only a small portion is preserved. Both the lateral and medial condyles are also partially preserved.

The tibia of A. lucasi is morphologically distinct from A. fra­

gilis. The medial condyle is ovoid shaped. As in other large- bodied non-avian theropods the condyles in A. lucasi are separated from each other by a deep and narrow intercondy- lar notch. The medial condyle is confluent with the cnemial crest; however, it is not clear whether the dorsal articular surface of the tibia was inclined distolaterally as in Acrocan­

thosaurus atokensis or the medial condyle was at the same level as the anterior end of the tibia as in A. fragilis. The lateral condyle is also ovoid shaped and of similar size as the medial condyle. In proximal view the lateral condyle is ex- tended more distally and appears slightly longer than the medial condyle. The lateral condyle is separated from the proximal end of the tibia by a well-developed, but shallow crescentic concavity, the incisura tibialis (Madsen, 1993;

Azuma, Currie, 2000; Madsen, Welles, 2000; Rauhut, 2003;

Benson, 2009). Although the lateral condyle is incomplete, it is tenable that on anterior edge of the condyle there was a well-developed distinct craniolateral projection, a feature that is characteristic of basal tetanuran theropods (e.g., Allo iss

ds

10 cm

Fig. 16. Left distal ischium in lateral view of Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

ds – distal symphysis, iss – ischial shaft

A

E

D C

B

cnc

cnc

cnc lc

lc

bcnc

int lc

icn

icn

mc

mc

10 cm

Fig. 17. Left proximal tibia of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma

County, Colorado, USA

A. Lateral. B. Medial. C. Anterior. D. Posterior. E. Dorsal views. Abbreviations:

cnc – cnemial crest; bcnc – base of the cnemial crest; icn – intercondylar notch; mc – medial condyle; lc – lateral condyle; int – incisura tibialis

saurus , Saurophaganax, and Torvosaurus). The craniolateral projection is also present in neoceratosaurs (e.g., Ceratosau­

rus and Elaphrosaurus), small coelurid theropods (e.g., Coe­

lurus, Ornitholestes, and Tanycolagreus), and tyrannosaurids (e.g., Albertosaurus, Gorgosaurus, Lythronax, Tarbo saurus, and Tyrannosaurus).

The cnemial crest in Allosaurus lucasi is incomplete;

however, as in other large-bodied, non-avian theropods (Benson, 2009) its base arises from the anterior surface of the tibia’s shaft and is curved anteroproximolaterally. The cnemial crest is elongated and narrowed transversely.

Pes. Allosaurus lucasi has a nearly complete and excep- tionally well-preserved left pes (Fig. 18). The pes consists of the following bones: four metatarsals and eight digital pha- langes, including one ungual phalanx.

Metatarsal I. The bone is missing a small portion of the most proximal end (Fig. 18). It is approximately 5.5 cm long. It is more massive than that of Allosaurus fragilis.

Both collateral ligament pits are well-preserved. As in other non-avian theropods the medial pit is much deeper and

slightly larger than the lateral pit. The medial condyle ap- pears more pronounced than the lateral condyle. Both con- dyles are separated from each other by a characteristic ging- lymus, which is approximately 1 cm deep. The maximum transverse width of the distal articular surface is approxi- mately 3.5 cm.

Metatarsal II. The bone is the best preserved and the most complete of all the metatarsals in Allosaurus lucasi (Fig. 19). The length of the entire bone is 33.5 cm. The bone is robust, straight, and has a uniformly wide shaft. The flat medial surface of the metatarsal shaft indicates that the bone was closely appressed to the metatarsal III for most of its length. The proximal articulation is typical for a theropod (Carrano, 2007), which is flat and featureless. The medial and lateral sides of the proximal articulation are missing a small portion of the bone. When reconstructed, the shape of the proximal articulation resembles to some extent that of Torvosaurus tanneri (Britt, 1991). The distal end of the shaft is bent slightly forward as in Acrocanthosaurus atokensis,

10 cm

Fig. 18. Associated left pedal of Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

A

B

5 cm

edls

II II

III IV

III IV

Fig. 19. Articulated left metatarsals of Allosaurus lucasi sp. nov.

(YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

A. Anterior. B. Distal views. Abbreviations: edls – fossa for insertion of m. extensor digitorum longus

Allosaurus fragilis, Megalosaurus bucklandii, and Torvos­

aurus tanneri. The distal articulation has well-preserved col- lateral ligament pits that are relatively deep. The medial pit is much deeper and larger than the lateral pit. As in other basal and derived tetanuran theropods the medial condyle in Allosaurus lucasi is flattened and much larger than the lat- eral condyle, which is acuminate. The height of the lateral condyle is 4.5 cm, whereas the height of the medial condyle is 6.5 cm. The distance between the condyles when meas- ured from their apices is 5 cm. In distal view the condyles are separated from each other by a deep fossa. In posterior view the condyles are separated by a deep ligament fossa, which has a depth of 2.5 cm. The much larger medial con- dyle articulates with the corresponding phalanx II-1. The an- terior face of the distal articulation has a well-preserved shallow fossa, which is interpreted as the insertion site of M.

extensor digitorum longus (Carrano, 2007). The posterior face of the metatarsal has a longitudinally striated facet that is located approximately at the mid-section of the bone. The facet represents the insertion for the M. gastrocnemius pars medialis (Carrano, Hutchinson, 2002; Carrano, 2007). Adja- cent to the proximal end of the insertion site for the M. gas- trocnemius pars medialis is a small depression that repre- sents the articulation for metatarsal I (Tarsitano, 1983).

Metatarsal III. The metatarsal III is largest bone in the foot (Fig. 19). The bone is missing the proximal end. In the most proximal region the shaft is round. The lateral sides of the shaft are flattened towards the posterior surface of the bone forming a V-like cross section. As in most other thero- pods (Snively et al., 2004) the shaft is medially deflected.

The medial face of the shaft has a large flat facet for the ar- ticulation with metatarsal II. The facet occupies approxi- mately two-thirds of the length of the bone. A prominent ridge extends distally down along the anteromedial edge of the metatarsal shaft. The lateral facet for the articulation with metatarsal IV is also pronounced and of similar length as that for metatarsal II. The distal articulation is broad transversely and roller-like and has a shallow fossa that sep- arates the condyles. Both condyles articulate with the corre- sponding phalanx III-1. Both collateral ligament pits are pre- served, equally deep, and circular shaped. The anterior face of the distal articulation preserves a deep pronounced fossa for insertion of the M. extensor digitorum longus (Carrano, 2007), which is mediolaterally wide. The posterior face of the distal articulation has a deep fossa that separates the two condyles, a feature that is common in tetanuran theropods (Carrano et al., 2012).

Metatarsal IV. This bone is missing the proximal end and a portion of the proximal shaft (Fig. 19). When recon- structed it is approximately the same length as the metatarsal II. The shaft is D-shaped in cross section as in most other large-bodied basal and derived tetanuran theropods and also

in abelisaurids (Carrano, 2007). The medial surface of the shaft is flat for most of its length, which indicates that the bone was closely appressed to metatarsal III for most of its length. The distal end of the metatarsal IV is not strongly diverged as in Allosaurus fragilis and A. jimmadseni. The lateral face of the shaft at its posterior region has a pro- nounced tuberosity that is located approximately 9 cm above the lateral condyle.

The anterior face of the distal end of metatarsal preserves a shallow fossa for the insertion of the M. extensor digito- rum longus. The fossa is more laterally oriented than anteri- orly. Two other shallow depressions extend along the pos- teromedial face of the shaft. These characteristic depressions are interpreted as the insertion sites for the M. gastrocnemius pars lateralis (Carrano, Hutchinson, 2002).

Near the distal articulation the shaft is slightly narrow.

The distal articulation is a small, D-shaped structure. The length/width ratio of the distal articular surface is 1.2. Both collateral ligament pits are preserved. The medial collateral ligament pit is a shallow depression; however, it is much deeper than the lateral ligament pit. In posterior view of the distal articulation, a shallow fossa separates the condyles.

Phalanges. Almost all of the phalanges of the left pes of Allosaurus lucasi are preserved (Fig. 20). The only phalan- ges that are missing are I-1 and I-2, and phalanx IV-4. Ad- ditionally, a single partial ungual phalanx of the pedal digit IV is preserved, whereas the unguals for digits I, II, and III are missing.

Phalanx II-1 is well-preserved. The length of the phalanx is 13 cm. The proximal articulation is larger in surface area than the distal articulation. The ventral side of the bone is damaged. The shaft is relatively short. The distal articulation preserves both condyles: medial and lateral. Anteriorly, the condyles are separated by a deep fossa that forms a charac- teristic ginglymus, as in other basal tetanuran theropods. The depth of the sulcus is approximately 2 cm. Both collateral ligament pits are well-preserved. The medial ligament pit is deeper than the lateral, as in most theropods. The hyperex- tensor pit is large, round, and well defined.

Phalanx II-2 is complete and well-preserved and has the length of approximately 8 cm. The proximal articulation is round and featureless and resembles phalanx II-1. The shaft is short and round. The transverse width of the distal end of the phalanx is nearly the same as the proximal end. Both col- lateral ligament pits are preserved. The medial pit is pushed inward, which makes it deeper than it might have been origi- nally. Both condyles, lateral and medial, are pronounced, and are separated by a deep sulcus.

Phalanx III-1 presents only the proximal end. The proxi-

mal articulation is round and has a shallow depression for

the articulation with the distal end of metatarsal III.

II

II

III

III

IV

IV

A

E D

C B

10 cm hp

clp

Fig. 20. Left pedal phalanges of Allosaurus lucasi sp. nov. (YPM VP 57589) from the Morrison Formation (Tithonian) McElmo Canyon, Montezuma County, Colorado, USA

A. Dorsal. B. Lateral. C. Ventral. D. Proximal. E. Distal views. Abbreviations: clp – collateral ligament pit; hp – ‘hyperextensor’ pit

Phalanx III-2 is complete and exceptionally well-pre- served and has a length of 8 cm. The proximal articulation is wider than high and is D-shaped. No visible ornamentation of any kind can be seen on the outer surface of the bone. The proximal articular surface has two shallow fossae for the ar- ticulation with phalanx III-1. The shaft is round and short, and wider than high. The transverse width of the distal ar- ticulation is the same as the proximal articulation. The con- dyles of the distal articulation are of equal sizes and are separated from one another by a shallow sulcus with a depth of 0.5 cm. Both collateral ligament pits are preserved, but are filled with hard sandstone matrix. The hyperextensor pit in the dorsoanterior region of the phalanx is extensive, deep and transversely wide.

Phalanx III-3 is proportionally short compared to pha- lanx III-2 and flat ventrally. The proximal articular surface is D-shaped and has two shallow fossae for the articulation with phalanx III-2. As the other phalanges, the outer surface of the proximal articulation of phalanx III-3 lacks any kind of ornamentation. The shaft is extremely short and the con- dyles of the distal articulation are almost making contact with the proximal end of the phalanx. The condyles are sepa- rated by a deep sulcus with a depth of 0.5 cm. Both collat- eral ligament pits are preserved. The medial pit is filled with hard matrix. The dorsal surface of the bone is crushed.

Phalanx IV-1 is exceptionally well-preserved and has a length of 8 cm. The proximal articulation is round, but its lateral side is skewed medially, as in other large-bodied non- avian theropods. The ventral surface of the proximal articu- lation is concave. The medial condyle is two times larger than the lateral condyle. The condyles are separated by a 1.5 cm deep sulcus. Both collateral ligament pits are pre- served in the phalanx. The lateral pit is only partially pre- served, and the medial ligament pit is filled with hard sand- stone matrix. The shaft of the phalanx is short. The hyperextensor pit is small and shallow.

Phalanx IV-2 is proportionally shorter than phalanx IV-1.

The shaft is extremely short, and the proximal and distal ends of the phalanx almost contact each other. The proximal articulation is D-shaped. The medial condyle is larger than the lateral. Both collateral ligament pits are preserved in the phalanx. The medial pit is filled with hard sandstone matrix.

The lateral pit is partially preserved because most of the dor- sal surface of the lateral condyle is missing. However, the remaining part of the pit suggests that it was not deep.

The dorsal region of the distal articulation is crushed.

Phalanx IV-3 is also proportionally smaller than the pre- vious phalanx and is approximately 6 cm long. The proximal and distal articulations have the same transverse widths. The proximal articulation is heavily damaged, especially its dor- sal surface. The overall shape of the proximal articular sur- face is a D-shape. Both condyles are preserved in the pha-

lanx. However, the medial condyle is only partially preserved and most of its dorsal surface is missing. The condyles are separated by a deep sulcus with a depth of 1 cm. Only the lateral collateral ligament pit is preserved.

The ungual phalanx of digit IV is only partially pre- served, missing the entire anterior region. The estimated length of the preserved bone fragment is 3 cm. Most of the bone surface is damaged, and no measurements can be ob- tained except for its general dimensions. In the dorsoposte- rior region, the bone preserves a relatively long dorsal pro- cess. The length of the process is approximately 1.5 cm.

DISCUSSION

Allosaurus lucasi represents the fourth unambiguous species of the North American Late Jurassic Allosaurus. The genus is the most abundant and the best known large-bodied basal tetanuran theropod in the world and is found through- out the entire Morrison Formation in Arizona, Colorado, Montana, New Mexico, South Dakota, Utah, and Wyoming (Gilmore, 1920; Smith et al., 1999; Brusatte, Sereno, 2008;

Carrano et al., 2012). Allosaurus is largely represented by subadult and adult examples; however, young juveniles are also known, but are scarce (Madsen, 1993; Foster, 2003, 2007; Foster, Chure, 2006). Until now three species of Allo­

saurus were known in the Morrison Formation, Allosaurus atrox, A. fragilis and A. jimmadseni (Marsh, 1878; Madsen, 1993; Chure, 2000; Heckert et al., 2003; Loewen, 2004, 2009). These taxa had the same temporal occurrence and most likely shared the same ecosystems (Loewen, 2004).

The genus is also present in Andrés, Praia de Vale Frades and Guimarota, Leiria, Alcobaça and Porto Novo Members, Lourinhã Formation (Kimmeridgian–Tithonian) of Portugal and is referred to a new species A. euro paeus by Mateus et al. (2006), which suggests that Allosaurus was both taxo- nomically and morphologically more diverse than previous- ly believed. However, other workers (Malafaia et al., 2007, 2010) suggested that A. europaeus is morphologically more similar to A. fragilis, and thus, may belong to this taxon.

Therefore, more detail studies of the Allosaurus materials from Portugal are still needed.

The identification of Allosaurus lucasi as a new species of Allosaurus not only supports the evidence for the diversi- ty of this genus in North America, but also suggests that the genus was present at the very end of the Jurassic.

The Morrison Formation is mostly Kimmeridgian and Early Tithonian in age (Foster, 2003).

A contemporary of Allosaurus lucasi is a large allosau-

roid Saurophaganax maximus (Ray, 1941; Chure, 1995)

from the Stovall Quarry 1, east of Kenton, Cimarron County,

Oklahoma. Both taxa have been found at the top of the Mor-

rison Formation (Tithonian). However, since its discovery the validity of Saurophaganax has been debated (Carrano et al., 2012). Some workers (e.g., Paul, 1988; Smith, 1998;

Holtz et al., 2004) referred it to a large Allosaurus maximus.

Chure (1995) identified two apomorphies on the skeleton of Saurophaganax, which validate its taxonomic status. Re- cently the validity of Saurophaganax was supported by the phylogenetic analysis of Carrano et al. (2012). In 1995, a partial skeleton of Saurophaganax consisting of several proximal caudal vertebrae, a partial right ilium, ischium and a nearly complete left hind limb was discovered at the Peter- son Quarry in New Mexico (Lucas, 1993, 2009, 2014), thus suggesting a much larger paleogeographic occurrence of this rare theropod.

Although Allosaurus lucasi and Saurophaganax maxi­

mus are largely fragmentary, they most likely do not repre- sent a single species. There is minimal overlapping material between both taxa, and some of the overlapping bones are incomplete or poorly preserved. A. lucasi has a more com- plete skull, whereas the skull of S. maximus is unknown, and only a single isolated right postorbital, two partial quadrates, and three poorly preserved tooth crowns are preserved. Both taxa are also of different body lengths: A. lucasi approxi- mately 10 m or slightly longer and S. maximus 14 m. How- ever, overall body size may be of little value, especially when dealing with similar morphology (Chure, 1995). But, both taxa can be distinguished from one another on the basis of several morphological characters, including the follow- ing: A. lucasi, a tall quadrate, strongly removed posteriorly lateral condyle of tibia, shallow but extensive incisura tibia- lis, whereas S. maximus, large lateral condyle of tibia that is more centered and occupies almost half the length of the tibial head. Both taxa exhibit a slightly divergent fourth met- atarsal. Further, A. lucasi can be also distinguished from A. fragilis and A. jimmadseni by a relatively short premaxilla and deep maxilla, which may have resulted in a much short- er and deeper skull in A. lucasi (Pl. 1: 1, 2). Another charac- teristic that distinguishes A. lucasi from both these taxa is the position of the antorbital fossa relative to the lateral al- veolar margin. In A. lucasi the fossa was situated much high- er and was not as extensive as in A. fragilis and A. jimmad­

seni, which extended nearly to the lateral alveolar margin.

The overall length of the more complete maxilla of A. lucasi was most likely similar to adults of A. fragilis (Carpenter, 2010) from the Cleveland Lloyd Quarry, Utah. However, the anterior portion of the maxilla of A. lucasi is much deeper than of A. fragilis. Premaxillary length reduction has also been observed in neoceratosaur theropods such as the North American Ceratosaurus, and in the majority of abelisaurid theropods such as Abelisaurus comahuensis, Aucasaurus garridoi, Carnotaurus sastrei, Rugops primus, and Skorpio­

venator bustingorryi (Bonaparte, Novas, 1985; Bona parte et

al., 1990; Coria et al., 2002; Sereno et al., 2004; Canale et al., 2008). The aliorami tyrannosaurids such as the most re- cently described Qianzhousaurus sinensis (Lü et al., 2014) pushed the length reduction of the pre maxillae to the ex- treme.

The quadratojugal of Allosaurus lucasi is very character- istic and differs from A. fragilis and A. jimmadseni. It is more robust and has a short jugal process and short quadrate process of the quadratojugal.

Allosaurus lucasi may be the stratigraphically youngest representative of the genus Allosaurus, which provides evi- dence for the diversity of allosauroid theropods during the Late Jurassic/Early Cretaceous in North America. This sug- gests that Allosaurus-like theropods were still present at the very end of the Jurassic and may have continued into the Cretaceous. However, more specimens are still waiting to be discovered to see how far the genus continued on its evolu- tionary path. A. lucasi provides further evidence of a distinc- tive upper Morrison dinosaur fauna, which was first sug- gested by Bakker (1986) and later supported by the discovery and description of a diplodocoid sauropod Suuwasea emi­

lieae (Harris, Dodson, 2004) from the uppermost Morrison Formation of Montana.

Acknowledgments. I wish to thank Daniel L. Brinkman and Christopher Norris for access to the specimens. I thank Dr. Spencer G. Lucas for inviting me to contribute to this special volume of Volumina Jurassica. Special thanks go to Joseph Lyons for taking the photograph of all the specimens.

I thank Dr. John Foster, Dr. Adrian Hunt, and an anonymous reviewer for the reviews and comments on the final version of this paper, which greatly improved its content. I wish to thank Dr. Roger B.J. Benson, Daniel L. Brinkman, Dr. Steve L. Brusatte, Dr. Ken Carpenter, Denver Fowler, Dr. Peter Galton, and Dr. Spencer G. Lucas for providing important literature on theropod dinosaurs, which greatly aided in this research. Lastly, I wish to thank the late Dr. Joseph T. Greg- ory and his field crew Robert Clem, A. Richard Diebold Jr., Jean Dudley, David Parsons, David Techter, and John T.

Walker, for the discovery of this important specimen. This work honors them both and their work continues to inspire.

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